Scientists thought evolutionary diversity enabled dinosaurs to reign

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Dinosaurs often are seen as unlucky, having been wiped out by an asteroid. But they dominated Earth for more than 160 million years, evolving into a wild array of body types and sizes suited for many different ecological niches.

Scientists previously thought that it was this evolutionary diversity that enabled the dinosaurs' reign, allowing them to out-compete similar groups of reptiles, but a new study, detailed in the journal Science, shows that it was really just a matter of luck.

"For a long time it was thought that there was something special about the dinosaurs that helped them become more successful during the Triassic, the first 30 million years of their history, but this isn't true," said lead author of the study, Steve Brusatte, a Ph.D. student at Columbia University and affiliate of the American Museum of Natural History in New York.

The closest competitors to the dinosaurs during the Triassic period (about 251 to 199 million years ago) were the crurotarsans, the ancestors of today's crocodiles.

Both dinosaurs and crurotarsans evolved and filled some of the same ecological niches after a massive extinction event at the end of the Permian period some 250 million years ago. Both groups also survived a later extinction event about 228 million years ago.

But only the dinosaurs (and crocodiles) made it through a period of rapid global warming at the end of the Triassic 200 million years ago. And avian dinosaurs are still with us today in the form of modern birds, which evolved from theropod dinosaurs and survived a separate and later mass extinction event at the end of the Mesozoic Era.

Special or lucky?
To see if there was indeed something "special" about the non-avian dinosaurs that allowed them to out-compete the crurotarsans, Brusatte and his colleagues compared the reptiles' rates of evolution and morphological disparities — or range of different body plans. The team used a database of 437 features from 64 skeletons and relied on a new family tree of the group Archosauria, which includes non-avian dinosaurs and crurotarsans, as well as pterosaurs, modern birds and crocodilians.